When you use まるい (the adjective for 'round') as an adverb, it becomes まるく
まるくなる：to become round

In other words, the cat is curling up.
I'm not sure where exactly the cat is curling up... beside the kotatsu, I suppose, since I would expect another construction besides simply marking with で if it was on top or under the kotatsu, and also beside the kotatsu is a good place for a cat to curl up and keep cozily warm without being -too- warm.

SomeCallMeChris Sensei
thank you. That is exactly what the English translation come out to be. you really solved the "curling up" mystery for me.

SomeCallMeChris wrote:When you use まるい (the adjective for 'round') as an adverb, it becomes まるく
まるくなる：to become round

In other words, the cat is curling up.
I'm not sure where exactly the cat is curling up... beside the kotatsu, I suppose, since I would expect another construction besides simply marking with で if it on top or under the kotatsu, and also beside the kotatsu is a good place for a cat to curl up and keep cozily warm without being -too- warm.